Electron-doped High-Tc Superconductors

The electron-doped cuprates, e.g. Pr_2-xCe_xCuO₄ (PCCO) and Nd_2-xCe_xCuO₄ (NCCO), possess the essential building block encountered in the cuprate family : the copper-oxygen planes. However, because of their particular crystal structure, they can only be doped with electrons (and not holes as for the other cuprates). That makes them very interesting for testing the mechanism of high-Tc superconductivity, assuming of course that the hole-doped and the electron-doped cuprate mechanisms have the same origin. Interestingly, several properties have been observed to be different.

The electrical resistivity of the normal state of electron-doped cuprates is known to follow a T² behavior from T_c up to room temperature. Moreover, for the optimal doping maximizing T_c, the resistivity is metallic for both the ab-plane and c-axis resistivity. These particularities distinguish them from the hole-doped cuprates, which show linear resistivity and non-metallic c-axis resistivity. Similar to the hole-doped cuprates, the electron-doped cuprates possess a strongly temperature-dependent Hall coefficient. However, contrary to the hole-doped materials, the Hall coefficient (and other transport coefficients) clearly indicates the contribution of two types of carriers, electrons and holes, to the normal state properties.

Another important issue is the nature of the superconducting state. For hole-doped cuprates it has been shown that the superconducting order parameter (gap) has a d-wave symmetry, quite different that most conventional, low-Tc superconductors which have a s-wave symmetry. Early experiments on NCCO suggested that the symmetry was s-wave in the electron-doped cuprates. However, recent penetration depth, specific heat and Raman experiments at the CSR have shown the gap symmetry in the electron-doped cuprates is d-wave (actually, non-monotonic d-wave).

Another important question about the high-T_c cuprates is the nature of the normal state at T=0, i.e. is it a conventional Fermi liquid or not. This question is difficult to answer in the hole-doped cuprates because it would take an enormous magnetic field to suppress the superconductivity at T=0 and reach the normal state. However, the electron-doped cuprates have a much lower magnetic critical field (H_c2 ~ 10 Tesla). Therefore, it has been possible to measure resistivity and Hall effect down to very low temperatures in the normal state as a function of Ce doping. These experiments give strong evidence for a quantum phase transition at a critical doping near Ce = 0.165 (Fig.1). Other experiments, such as neutron scattering and optics, show that this is an Antiferromagnetic to Paramagnetic transition.This work will put constraints on theories which attempt to explain the origin of high-T_c superconductivity.

Other experiments that we have carried out recently on the electron-doped cuprates include Raman scattering (Fig.2), Nernst effect (Fig.3), Heat capacity (Fig.4), and tunneling (Fig.5). All these experiments are providing more understanding of the normal state and superconducting state of high-T_c superconductors.

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